Process for desalting crude oil

ABSTRACT

The invention provides a process for desalting crude by combining water and raw crude oil in a mixer to produce a mixture. Desalted crude oil and a brine mixture are separated from the mixture. The water is obtained as a byproduct of a Fischer-Tropsch plant.

PRIOR RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser. No. 60/764,702, filed on Feb. 2, 2006, which is hereby incorporated in it's entirety.

FEDERALLY SPONSORED RESEARCH STATEMENT

Not applicable.

REFERENCE TO MICROFICHE APPENDIX

Not applicable.

FIELD OF THE INVENTION

The invention relates to a desalting process for crude oil. More specifically, the invention relates to a desalting process for crude oil that uses water produced from a Fischer-Tropsch plant.

BACKGROUND OF THE INVENTION

Crude oil entering a petroleum refinery contains a number of impurities harmful to the efficient operation of the refinery and detrimental to the quality of the final petroleum products. Some impurities include, but are not limited to, salts of magnesium, sodium and calcium, potassium, nickel, vanadium, copper, iron and zinc, and are unstable at elevated temperatures. If allowed to remain in the crude oil, the salts contribute to corrosion in the main fractionator unit and other regions of the refinery system where temperatures are elevated, as well as any area where water condenses. These impurities also contribute to heat exchanger fouling, furnace coking, catalyst poisoning and end product degradation.

Crude oil desalting removes most of the impurities and is a common emulsion breaking method where the emulsion is first intentionally formed. Water is added in an amount of approximately between 5% and 10% by volume of crude. The added water is intimately mixed with the crude oil to contact the impurities therein, thereby transferring these impurities into the water phase of the emulsion. The emulsion is usually resolved with the assistance of emulsion breaking chemicals, which are characteristically surfactants. Alternatively, the emulsion may be resolved by application of an electrical field to polarize the water droplets. Once the emulsion is broken, the water and petroleum media form distinct phases. The water phase is separated from the petroleum phase and subsequently removed from the desalter. The petroleum phase is directed further downstream for processing through the refinery operation.

As oil refineries move to heavier crudes having higher inorganic salt content, the performance of the desalter is becoming increasingly important. Therefore, a clean water supply, free from metals and salts, which will improve the performance of the desalter is needed.

SUMMARY OF THE INVENTION

Large amounts of water, with very low inorganic content, are produced at a Fischer-Tropsch plant. The water normally has to be processed for discharge into the environment or transported offsite for disposal. Using the Fischer-Tropsch byproduct water in a desalting process unit would utilize existing crude desalting infrastructure facilities and utilities, lessen the environmental impact of the processing units and provide “cleaner” water to the desalter process thereby increasing desalter importance.

Embodiments of the invention provide a process to desalt crude oil utilizing the water byproduct from a Fischer-Tropsch plant. Embodiments of the invention provide a process for desalting crude by combining water and raw crude oil in a mixer to produce a mixture. Desalted crude oil and a brine mixture are separated from the mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of an embodiment of an inventive crude desalting method.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Unless otherwise specified, all quantities, percentages and ratios herein are by weight.

The term “desalted crude oil” means a crude oil which following desalting treatment has less than 70% of the salt content present in the crude oil prior to desalting, or may be less than 75%, less than 80%, less than 90%, less than 95%, and most preferably 99.75% of the salt content present in the crude oil prior to desalting.

The term “brine” means an aqueous stream containing inorganic salts.

The term “FT water” means an aqueous stream which is produced by a Fischer-Tropsch plant.

Referring to FIG. 1, a desalting unit 10 is described. Crude oil 12 and water 14 are fed into a static mixer 16. An emulsion 18 is formed and fed to a settling tank 20. Products from the settling tank 20 include a desalted crude oil 22 and a brine mixture 24. The conditions of the static mixer are around 250° F. (121.1° C.) and pressures high enough to prevent vaporization of either water or hydrocarbons. These conditions will vary depending on the make-up of the crude oil 12 and water 14.

The water 14 fed to the static mixer 16 is from a Fischer-Tropsch (FT) plant, “FT water”. FT plants produce about as much FT water as hydrocarbon product. Because the FT water originates from a process in which the feedstock is a natural gas, the FT water is virtually free of minerals and/or salts.

An FT plant for converting hydrocarbon gases to liquid or solid hydrocarbon products includes a synthesis gas unit, which includes a synthesis gas reactor in the form of, for example, an autothermal reforming reactor (ATR) containing a reforming catalyst, such as a nickel-containing catalyst. A stream of light hydrocarbons to be converted, which may include natural gas, is introduced into the reactor along with oxygen (O₂). The oxygen may be provided from compressed air or other compressed oxygen-containing gas, such as oxygen enriched air, or may be a pure oxygen stream. The light hydrocarbon stream may also arise from gasified coal. The ATR reaction may be adiabatic, with no heat being added or removed from the reactor other than from the feeds and the heat of reaction. The reaction is carried out under sub-stoichiometric conditions whereby the oxygen/steam/gas mixture is converted to syngas. Examples of Fischer-Tropsch systems are described in U.S. Pat. Nos. 4,973,453; 5,733,941; 5,861,441; 6,130,259, 6,169,120 and 6,172,124, the disclosures of which are herein incorporated by reference.

Techniques for producing a synthesis gas, or syngas, which is used as the starting material of a Fischer-Tropsch reaction are well known in the art and include oxidation, reforming and autothermal reforming. The Fischer-Tropsch reaction for converting syngas, which is composed primarily of carbon monoxide (CO) and hydrogen gas (H₂) may be characterized by the following general reaction: 2nH₂ +nCO→(—CH₂—)n+nH₂O  (1)

Non-reactive components, such as nitrogen, may also be included or mixed with the syngas. This may occur in those instances where air or some other non-pure oxygen source is used during the syngas formation.

The syngas is delivered to a synthesis unit, which includes a Fischer-Tropsch reactor (FTR) containing a Fischer-Tropsch catalyst. Numerous Fischer-Tropsch catalysts may be used in carrying out the reaction. These include cobalt, iron, ruthenium as well as other Group VIIIB transition metals or combinations of such metals, to prepare both saturated and unsaturated hydrocarbons. For purposes of this invention, a non-iron catalyst may be used. The F-T catalyst may include a support, such as a metal-oxide support, including silica, alumina, silica-alumina or titanium oxides. For the purposes of this reaction, a Co catalyst on transition alumina with a surface area of approximately 100-200 m²/g is used in the form of spheres of 50-150 μm in diameter. The Co concentration on the support may also be 15-30%. Certain catalyst promoters and stabilizers may be used. The stabilizers include Group IIA or Group IIIB metals, while the promoters may include elements from Group VIII or Group VIIB. The Fischer-Tropsch catalyst and reaction conditions may be selected to be optimal for desired reaction products, such as for hydrocarbons of certain chain lengths or number of carbon atoms. Any of the following reactor configurations may be employed for Fischer-Tropsch synthesis: fixed bed, slurry reactor, ebullating bed, fluidizing bed, or continuously stirred tank reactor (CSTR). For the purposes of this reaction, a slurry bed reactor is used. The FTR may be operated at a pressure of 100 to 550 psia (689 to 3792 kPa) and a temperature of 350° F. to 500° F. (176.6 to 260° C.). The reactor gas hourly space velocity (“GHSV”) may be from 1000 to 8000 hr−1. Syngas useful in producing a Fischer-Tropsch product useful in the invention may contain gaseous hydrocarbons, hydrogen, carbon monoxide and nitrogen with H₂/CO ratios from about 1.8 to about 2.4. The products derived from the Fischer-Tropsch reaction may range from methane (CH₄) to high molecular weight paraffinic waxes containing more than 100 carbon atoms and water.

In an embodiment of the invention, the FT plant and the desalting process are located at an oil-producing field. Such field may be located on shore, near shore or off shore. The associated gas from the oil-producing field is converted to hydrocarbons in the FT plant and the byproduct water from the FT plant, FT water, is sent to the desalting unit 10. The associated gas typically contains 92 mol % methane, 3 mol % ethane, 2 mol % propane, 0.5 mol % butanes, and 2.5 mol % C₅-C₈ paraffins but may vary depending on the oil-producing field. The desalted crude oil 22 can be sent for further processing in the field or sent to a refinery. After processing at the oil field, the brine mixture 24 can be re-injected into the oil-producing field. If the crude is desalted at the oil field, the desalted crude can bypass the existing desalter at the refinery, which will provide capacity increase opportunities for desalter-limited refineries.

In an alternate embodiment, the FT plant is located at an oil refinery. The desalter in a refinery is typically located upstream of the atmospheric fractionator (sometimes called a “crude unit”). The pressures and temperatures in the crude unit are between 10-50 psig (68.9-344 kPa) and 200-750° F. (93.3-398.8° C.). The FT plant and refinery can be integrated to utilize existing refinery and utilities infrastructure. The brine mixture 24 can be processed in existing refinery facilities. This integration provides an opportunity to expand the refinery crude desalting capacity independent of the water demineralization facilities.

In a preferred embodiment, the static mixer 16 and the settling tank 20 are parts of the desalting unit 10. In alternate embodiments, the desalting unit 10 is any common desalting unit that uses water.

In an alternate embodiment, the crude oil 12 and FT water 14 are preheated before entering the static mixer 16. In another embodiment, a demulsifying surfactant is added to the settling tank 20. The slight acidity of the FT water 14 (pH typically between 3-5), coming from dissolved acid byproducts of FT synthesis, is expected to help with demulsification. As such, the demulsifying surfactants may not be needed or required at significantly smaller concentrations.

In yet other embodiments of the invention, other equipment for producing an oil/water emulsion may be used in lieu of static mixer 16. Such other types of equipment include, for example, pressure-reducing valves, continuous-flow stirred tanks with side-entering or top-entering propeller mixers, in-line turbine agitators, or jet mixers. Any alternative mixing method and/or apparatus may be used so as to achieve the formation of the emulsion. Since desalting is a mass-transfer limited process, the higher the water-oil contact area, the better the performance. This means that the emulsion droplets need to be small enough to provide a high surface area for migration of salts from oil to water, but not so small that residence times required for coalescence in the settler become too long.

In other embodiments of the invention, other techniques and/or equipment for separating emulsions may be used in lieu of a surfactant and/or settling tank 20. For example, electrostatic precipitators, dehydrators, or cyclonic separators could alternatively be used to break the emulsion. Any method and/or process sufficient to achieve separation of the emulsion into an aqueous phase and an oil phase, wherein the oil phase contains no free water may be used.

In various embodiments of this invention, the desalting process may be located at a refinery, at a well site, or on a movable platform, such as a barge or ship.

Other sources of water for the desalting of crude can be found in a FT plant. In a preferred case, the FT plant is part of a Natural Gas-to-Liquids plant where the reformer also produces a water product (“ATR water”). The ATR water is also low in total dissolved solids (TDS) and will provide desalting capacity. Exemplary compositions of the FT water stream and the ATR water stream are in Table 1, but embodiments of the invention should not be limited by these. TABLE 1 Chemical analysis of water from FT Reactor and Autothermal Reformer FT ATR water water Relative production ratio 1 0.08 pH 4.2 7.89 Conductivity (mS) 180.5 9133 TDS (mg/L) 50.5 25 TSS, total suspended solids 0.0 3.0 (mg/L)

Also, in the hydroprocessing of FT synthesis products, the conversion of alcohols and, to a lesser extent, carboxylic acids, ketones and aldehydes, produce water. This “process water” is also very low in solids and, as such, will provide desalting capacity.

While the invention has been described with respect to a limited number of embodiments, the specific features of one embodiment should not be attributed to other embodiments of the invention. No single embodiment is representative of all aspects of the inventions. Moreover, variations and modifications therefrom exist. For example, other desalting process units can be used in place of the static mixer and settling tank. Additionally, heat exchangers and preheaters may be designed for maximum heat efficiency. The appended claims intend to cover all such variations and modifications as falling within the scope of the invention. 

1. A process for desalting crude comprising: a) combining water and raw crude oil in a mixer to produce a mixture; b) separating from the mixture a desalted crude oil and a brine mixture; wherein the water is obtained as a byproduct of a Fischer-Tropsch plant.
 2. The process of claim 1, further comprising preheating the water and/or the raw crude oil before combining them.
 3. The process of claim 1, further comprising adding a demulsifier to the mixer.
 4. The process of claim 1, wherein the process occurs at an oil producing site.
 5. The process of claim 1, wherein the process occurs at an oil refining site.
 6. The process of claim 4, further comprising re-injecting the brine mixture into a hydrocarbon formation at the oil producing site.
 7. The process of claim 1, further comprising processing the brine mixture for reuse.
 8. The process of claim 1, further comprising processing the brine mixture for discharge.
 9. The process of claim 1, wherein the process occurs on a movable platform.
 10. A process for desalting crude comprising: a) forming an emulsion of crude oil and Fischer-Tropsch water; and b) breaking the emulsion to obtain a desalted crude oil and a brine mixture.
 11. The process of claim 1 wherein the Fischer-Tropsch water contains less than 1% inorganic salts.
 12. The process of claim 1 wherein the Fischer-Tropsch water contains less than 0.5% inorganic salts.
 13. The process of claim 1 wherein the Fischer-Tropsch water contains less than 0.01% inorganic salts.
 14. The process of claim 1 wherein the desalted crude oil contains less than 40 wppb inorganic salts.
 15. The process of claim 1 wherein the desalted crude oil contains less than 10 wppb inorganic salts. 